Bootstrap/scss/_close.scss

.close {
  float: right;
  font-size: ($font-size-base * 1.5);
  font-weight: $close-font-weight;
  line-height: 1;
  color: $close-color;
  text-shadow: $close-text-shadow;
  opacity: .2;

  @include hover-focus {
    color: $close-color;
    text-decoration: none;
    cursor: pointer;
    opacity: .5;
  }
}

// Additional properties for button version
// iOS requires the button element instead of an anchor tag.
// If you want the anchor version, it requires `href="#"`.
// See https://developer.mozilla.org/en-US/docs/Web/Events/click#Safari_Mobile
button.close {
  padding: 0;
  cursor: pointer;
  background: transparent;
  border: 0;
  -webkit-appearance: none;
}